Is expiation obligatory upon the father if his son breaks an oath, having forgotten or been ignorant of it?

There is no expiation due upon your father if you did what he swore you not to do, while you had forgotten his oath or were unaware of it. This is because expiation is due to avert sin, and there is no sin upon either of them, and because Allah has pardoned this Ummah for their mistakes, forgetfulness, and what they are coerced into. And for His (Allah's) saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended."